The present invention relates to improved printed wiring board (PWB) thermal planes having a low coefficient of thermal expansion and high thermal conductivity. In particular, the present invention is directed to a graphite reinforced metal thermal plane for use with a PWB having a dissimilar thermally conductive material about the free edge surface of the composite.
PWB thermal plane assemblies are well known in the electronics industry. Such assemblies consist typically of PWB constructed of thermosetting resin matrix reinforced with fibrous material. The fibrous reinforced material is normally glass, although other dielectric reinforced materials such as quartz and aramid have also been used. The printed wiring board is completed by the creation of appropriate patterns of electrically conductive material on one or both surfaces of the board. The coefficient of thermal expansion and the thermal conductivity of the printed wiring board are two important characteristics in this formation. Large differences in the coefficient of thermal expansion between the printed wiring board and the components, solder and plating on the board will result in solder joint cracking when the assembly is subjected to temperature variation. Therefore, it is important to match the coefficient of thermal expansion (CTE) in the plane of the PWB to the CTE of the ceramic chip carriers or other chip carriers placed on the PWB. The thermal plane act to restrain the PWB given its lower CTE.
In addition, the PWB thermal plane assembly must have an acceptable thermal conductivity because convection cooling of electronic components is not possible or practical in many cases. It is therefore desirable to produce a PWB thermal plane assembly having a thermal conductivity high enough to transfer large heat loads which are generated during operation of the electronic components utilizing the PWB. The high thermal conductivity of the PWB, however, has to be achieved without sacrificing other important properties such as the appropriate CTE. In U.S. Pat. No. 4,609,586, an appropriate printed wiring board substrate is disclosed which comprises a metal matrix reinforced graphite fiber. While the printed wiring board of U.S. Pat. No. 4,609,586 has suitable thermal expansion and thermal conductivity properties for use in printed wiring boards, it may be improved upon. The present invention is directed to a metal matrix reinforced graphite fiber printed wiring board having improved properties.